1. Field of the Invention
The present invention relates to an absorbance monitor for measuring the concentrations of specific components in various fluids and is particularly useful in a highly accurate and compact fluid concentration measuring instrument for measuring a plurality of components present in a sample.
2. Description of the Related Art
In various production processes in recent years, it is essential to control the concentrations of substances controlled and various interfering substances, and highly accurate measuring instruments capable of coping with various uses are required. Conventionally, general-purpose absorbance monitors capable of measuring many kinds of components simultaneously in the same constitution, such as a non-dispersive infrared analyzer (referred to hereinafter as “NDIR”) and a non-dispersive ultraviolet analyzer (referred to hereinafter as “NDUV”), are often used. Such absorbance monitors do basically not contact a sample and are thus useful as in-line monitors in various processes.
Specifically, there is practically used a method of using a stable and multi-component measuring absorbance monitor illustrated in FIGS. 8(A) and (B) wherein optical elements 7a and 7b between a light source 1 and a detector 3 are arranged with a predetermined inclination relative to a light path, to diverge and introduce light into a plurality of detectors 3a, 3b and 3c such that a change in absorbance in a sample cell 2 can be detected (see, for example, Japanese Patent No. 2903457).
That is, when a power source (not shown) for the light source supplies electric power, an infrared ray from the light source 1 enters via the sample cell 2 into the detectors 3a, 3b and 3c. Optical filters 7a and 7b permitting an infrared ray in a wavelength range corresponding to a measurement component to penetrate selectively therethrough are arranged between the light source 1 and detectors 3a, 3b and 3c, and a change in the amount of only the infrared ray absorbed by a measurement component in a fluid sample introduced into the sample cell 2 is detected. A fluid switch 4 is used in modulation by periodically switching a fluid sample S and a standard (reference) fluid R with each other, and a change in the amount of the absorbed infrared ray in the sample cell is taken out as an alternating current signal, and this detector output is amplified with a preamplifier (not shown), then inputted into a signal processing unit (not shown), subjected to signal processing such as commutation and calculated to indicate the concentration of the component in an indicator (not shown).
In place of the fluid modulation system described above, there are known a mechanical light intermittence system wherein a chopper driven by a motor is arranged in the middle of the optical system to convert the infrared ray into an intermittent light to be introduced into a detector and a light modulation system NDIR wherein in place of the chopper, a light source voltage modulation means is arranged between a power source and a light source, to modulate electric power applied to the light source by turning on and off electric power. Using such constitution, not only measurement methods of using the NDIR but also measurement methods of using NDUV are proposed and practically used (see, for example, JP08-43302A).
In a sample containing a plurality of components, however, the respective components are often significantly different in concentration from one another, and the absorbance monitors described above in the related art may cause the following problems.
One problem lies in measurement errors caused generally by a relationship similar to light absorption characteristics called the Lambert-Beer law expressed in the following equation (1), between the quantity of light absorbed (that is, the concentration of component to be measured) and detector output.I0/I=A×log (εcd)  (1)where I0 and I represent the quantity of incident light and the quantity of penetrated light respectively in a sample cell, and ε, c and d represent light absorption coefficient, concentration of objective substance, and cell light path length, respectively.
That is, in the optical system using one sample cell, the quantity of light absorbed is significantly varied depending on the intended component so that when a low-concentration sample is measured, an output region of good linearity can be utilized, while when a high-concentration component is measured, the linearity of output is deteriorated, thus causing a difference in reading miscalculation and a difference in temperature characteristics. Accordingly, two or more optical systems different in cell length are often used in one analyzer, resulting in an enlargement of the analyzer.
When the number of components to be measured in the constitution shown in FIG. 8(A) is increased, the constitution in FIG. 8(B) is used and an additional optical element 7b should be used in this constitution. So due to optical loss caused by a larger size of the optical system and an addition of the part, there may arise a problem of deterioration in detection sensitivity.
For the purpose of reducing a sample flow in the optical system described above, the aperture diameter of the sample cell may be decreased, but the same problem may also arise in this case. Specifically, there is an optical system wherein a light collecting member 5 or a sample cell 2b is additionally used as shown in FIG. 9, but there may arise a problem of deterioration in detection sensitivity due to optical loss caused by addition of the part.